This invention relates to a continuous belt, drum type vacuum filter system and, in particular, to an improved apparatus for cleaning the drum surface and elements associated with the drum surface.
A drum type vacuum filter basically consists of a cylindrical drum covered with a filtering material or fabric which rotates partially submerged in a tank containing a slurry comprising solids and a liquid. Where the vacuum filter is used in a sewage treatment plant, the slurry will be a sludge of solid wastes and water. As the drum and belt rotate, the slurry will adhere to the outer surface of the belt and be carried on the belt along the surface of the drum out of the tank. The drum has a plurality of openings in its cylindrical surface and a vacuum is applied internally of the drum to extract water from the slurry. In being drawn into the drum by the vacuum, the water passes through pores or spaces in the belt and through the drum surface openings, leaving the solids on the belt. As the drum and belt continue rotating, the belt passes off or extends from the drum over relatively small rollers to dislodge the solids and again returns to the drum and passes into the tank. In addition, a scraper blade is typically positioned against the belt to insure that the solids are removed from the belt. Also to assist in flow of the water from the slurry through the belt and into the drum, means such as grids carried on the cylindrical drum surface and maintaining the belt at a spacing from the drum surface are used.
A serious problem with this type of filter system is that small particles of the solid in the slurry are drawn through the pores or openings in the belt and into the grid openings and the openings in the drum surface. These solid particles will plug the belt pores as well as the grid and drum surface openings so that the filter system becomes inoperative . The solid particles tend to harden in the grid and drum surface openings and become extremely difficult to remove. Cleaning of the grids and drum surface openings is quite difficult since most of the drum surface is covered by the belt. Moreover, attempting to clean the grids and the drum surface openings in the space formed by the drum and the extension and return of the belt from the drum is also quite difficult since the axial length of the drum and the width of the belt is considerable. For example, lengths of vacuum filter drums are typically between 20 and 30 feet.
Solutions to the problem have included the placing of an elongated pipe or spray header running the full axial length of the drum in the space formed by the drum and the extension and return of the belt from the drum. Spray nozzles are located in the spray header along its full length to spray water against the drum surface or against the belt to provide a cleaning effect. This approach is very ineffective and is not believed to be practiced to any great extent on commercial vacuum drum filter systems. Steam cleaning methods are also used in which the belt is removed from the drum, a large enclosure is placed over the drum and the drum is subjected to steam until the solids clogging the drum surface openings and grid openings disintegrate. Disadvantages of this type of cleaning include the necessity of removing the belt, the expense and problem of a large enclosure over the drum and the cost of a boiler for providing steam.
Other methods include manual cleaning which requires removal of the belt and the grids from the drum and manually cleaning the drum openings and the grids. Methods such as soaking the grids in any of a variety of cleaning baths and/or subjecting the grids and the drum surface to ultrasonic devices may also be utilized. The disadvantages of these methods also include the necessity of removing the belt and removing the grids. It can be appreciated that the foregoing described cleaning methods require the drum filter to be taken out of operation for a considerable length of time. Also, the amount of manual labor in removing the belt and cleaning the grids and drum surface is substantial. Further, removing the belts from the drum deteriorates the belts to the extent that they typically cannot be put back into service. The belts are quite expensive and the inability to continue their use adds a large expense to the operation of the sewage treatment plant.